Add experimental RA procedural map generator

Add an experimental procedural map generator for the Red Alert mod,
along with supporting code that may assist in the development of map
generators for other mods.

Map generation may be accessed as a tool in the Map Editor. This change
does not presently introduce direct lobby options for generated maps.

Features:

- Terrain with land, water, beaches, cliffs, roads, debris, and trees.
- Placement of mpspawns, neutral buildings, and resources.
- Rotational and mirror symmetry options.
- Various configurable parameters with presets.
- Deterministic with configurable seed.
- Performant.

OpenRA.Test/OpenRA.Game/PriorityArrayTest.cs

@@ -0,0 +1,66 @@
+#region Copyright & License Information
+/*
+ * Copyright (c) The OpenRA Developers and Contributors
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using NUnit.Framework;
+using OpenRA.Primitives;
+
+namespace OpenRA.Test
+{
+	[TestFixture]
+	sealed class PriorityArrayTest
+	{
+		[TestCase(0)]
+		[TestCase(5)]
+		[TestCase(int.MinValue)]
+		[TestCase(int.MaxValue)]
+		public void PriorityArraySequentialTest(int initialValue)
+		{
+			var input = new KeyValuePair<int, int>[]
+			{
+				new(0, 1),
+				new(1, 5),
+				new(2, 3),
+				new(3, 2),
+				new(4, 8),
+				new(5, 7),
+				new(6, 4),
+				new(7, 6)
+			};
+			var expected = new KeyValuePair<int, int>[]
+			{
+				new(0, 1),
+				new(3, 2),
+				new(2, 3),
+				new(6, 4),
+				new(1, 5),
+				new(7, 6),
+				new(5, 7),
+				new(4, 8)
+			};
+
+			var pa = new PriorityArray<int>(8, initialValue);
+
+			foreach (var kv in input)
+				pa[kv.Key] = kv.Value;
+
+			var readback = new KeyValuePair<int, int>[8];
+			for (var i = 0; i < 8; i++)
+			{
+				var index = pa.GetMinIndex();
+				readback[i] = new KeyValuePair<int, int>(index, pa[index]);
+				pa[index] = int.MaxValue;
+			}
+
+			Assert.That(readback, Is.EquivalentTo(expected));
+		}
+	}
+}